Why are the agricultural solar terms in China calculated according to foreign calendars?

First of all, it should be clear that China has used the lunar calendar to mark the twenty-four solar terms for thousands of years, but the dates have changed greatly and it is not easy to remember. After the Gregorian calendar was introduced into China, people began to remember it. After the liberation of the mainland, it is stipulated that the Gregorian calendar should be implemented, and the Gregorian calendar date of solar terms is relatively fixed, so everyone is willing to use the Gregorian calendar to remember solar terms.